1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of fabricating a liquid crystal display device, and more particularly, to an optically compensated birefringence mode liquid crystal display device and a method of fabricating an optically compensated birefringence mode liquid crystal display device.
2. Discussion of the Related Art
Among various types of liquid crystal display (LCD) devices commonly used, active matrix LCD (AM-LCD) devices in which switching elements, such as thin film transistors (TFTs), are disposed in a matrix configuration for each pixel region have been developed because of their high resolution and superiority in displaying moving images.
The LCD devices may use an electro-optical effect, wherein images are displayed by changing optical properties of a liquid crystal material layer due to a voltage applied to the liquid crystal material layer. The LCD devices that make use of the electro-optical properties can be classified into three types: current-effect type; electric field-effect type; and heat-effect type. The LCD devices of the electric field-effect type function in one of a twisted nematic (TN) mode, a guest-host (GH) mode, an electrically controlled birefringence (ECB) mode, and a phase change mode.
In the ECB mode, a liquid crystal material layer that is uniformly oriented is interposed between orthogonal polarizers, whereby light transmittance is changed according to the birefringence effect of the applied voltage. An LCD device of an optically compensated birefringence (OCB) mode, which is one of the ECB modes, has a symmetric bend structure such that an angle between a long axis of liquid crystal (LC) molecules and the substrate is nearly 90° toward a midway point between the upper and lower substrates, wherein the angle gradually decreases as the molecules approach the substrates. Therefore, the LCD device of the OCB mode has a relatively low response time.
FIG. 1 is a schematic perspective view of an optically compensated birefringence mode liquid crystal display device including a compensation film according to the related art. In FIG. 1, a bend cell 50 includes first and second substrates 10 and 30 spaced apart from each other and a liquid crystal material layer 40 disposed therebetween. The first and second substrates 10 and 30 may include respective orientation films having identical alignment directions. The liquid crystal material layer 40 has a splay structure when a voltage is not applied, and has a bend structure when a voltage that is above a transition voltage is applied. Accordingly, a time for realignment, i.e., a response time, is less than about 5 milliseconds.
However, the bend cell 50 uses a wide view film including a retarder to improve a viewing angle with high display quality. Accordingly, a first compensation film 12 and a first polarizing film 14 are sequentially formed on an outer surface of the first substrate 10, and a second compensation film 32 and a second polarizing film 34 are sequentially formed on an outer surface of the second substrate 30, wherein a first optical axis of the first compensation film 12 is perpendicular to a second optical axis of the second compensation film 32, and a first transmission axis of the first polarizing film 14 is perpendicular to a second transmission axis of the second polarizing film 34. The first compensation film 12 is a wide view film including a first discotic film 12a having a negative refractive index anisotropy (Δn) and a first biaxial film 12b sequentially formed on the outer surface of the first substrate 10. Similarly, the second compensation film 32 is also a wide view film including a second discotic film 32a having a negative refractive index anisotropy (Δn) and a second biaxial film 32b sequentially formed on the outer surface of the second substrate 30. The first and second biaxial film 12b and 32b have a birefringence property of ny>nx>nz.
When an OCB mode LCD device is fabricated, several factors, such as brightness, viewing angle, driving voltage, and response time, are considered. Exact design of the bend cell and the compensation film are especially important factors during fabrication of the OCB mode LCD device. Accordingly, several physical parameters of the liquid crystal material layer and design value of the compensation film determine performance of the OCB mode LCD device.
FIGS. 2A to 2C are schematic cross sectional views showing a driving principle of an optically compensated birefringence mode liquid crystal display device according to the related art. In FIG. 2A, a liquid crystal material layer has a splay structure when a voltage is not applied to the liquid crystal material layer. In FIG. 2B, the liquid crystal material layer has a bend structure when a transition voltage is applied. In FIG. 2C, liquid crystal molecules of the liquid crystal material layer are vertically aligned from the bend structure when voltages are applied. Therefore, a response of the OCB mode LCD device is faster than that of a TN mode LCD device.